1. Field of the Invention
The present invention relates to interference lithography techniques capable of generating periodic arrays of sub-micron sized structures. Specifically, wavefront division (WD) interference lithography is enhanced through the incorporation of a novel active stabilization technique. In addition, the disclosed invention relates to the incorporation of actively stabilized WD interference lithography in a flexible configuration, high-throughput, production-worthy system. Such a system finds countless applications in the telecommunications, display, biotechnology, semiconductor, and data storage industries.
2. Description of the Prior Art
Interference Lithography (sometimes referred to as holographic or interferometric lithography) is a proven technology used primarily in research and device prototyping laboratories. The main advantage of the technology is the ability to pattern sub-micron-sized features in arrays covering large areas.
Interference Lithography (IL) exploits the mutual coherence of multiple optical beams derived from a single light source such as a laser. The beams are made to overlap in some region of space where they interfere to produce patterns of light and dark areas which repeat on a scale proportional to the wavelength of the laser. The interference pattern exists throughout a volume of space and can be recorded by photosensitive media such as photoresist, placed within the overlap region. Conventional contact or projection photomasks are not required, yielding what has become known as a ‘maskless’ lithography technique. In addition, by exploiting inherent photoresist and etching process non-linearities, a variety of surface relief structures can be generated with no change in the optical configuration.
There are two main types of interference lithography practiced in the art. They are known as division-of-amplitude and division-of-wavefront, both of which are taught extensively by U.S. Pat. No. 4,496,216. Division-of-amplitude is most commonly practiced due to its greater control over the parameters governing the production of a stable, high contrast interference pattern. For example, one skilled in the art can readily adjust the positions, angles, phase, and polarization state of each interfering beam as described in U.S. Pat. No. 6,088,505, and the relative phases of the interfering beams can be locked with an active system as described in U.S. Pat. No. 5,142,385. Despite these advantages, division-of-wavefront IL is preferred when the application calls for extremely small feature sizes in the sub-half-micron range due to its simplified optical configuration, and the inherent high stability of the interference pattern. These advantages have not been fully realized in high-volume manufacturing environments since existing methods for actively stabilizing the interference patterns produced by a division-of-wavefront configuration, have not been practical. In addition, the use of division-of-wavefront IL in applications requiring the fabrication of structures which vary in two dimensions, has been limited due to the lack of a practical method for optimizing the interference pattern contrast, and the additional complexity of incorporating an active stabilization system which operates on more than one divided beam section.
Many applications would benefit from the sub-half-micron sized structures that can be produced using a division-of-wavefront interference lithography technique. A primary example is found in the production of gratings within the structure of semiconductor lasers used in optical telecommunications. Such light sources are known as distributed feedback lasers (DFB). They are typically constructed from indium phosphide (InP) material and are used extensively within wavelength division multiplexing, or WDM optical networks. These solid state lasers emit light in a multi-longitudinal mode—there exist numerous narrow band wavelengths clustered about the center wavelength. The grating incorporated within a DFB laser acts as a filter to limit the lasing output to a single narrow-banded mode important for high-speed telecommunications. Gratings are currently produced in one of three ways; 1) a phase mask technique, 2) e-beam lithography, or most commonly 3) interference lithography. The first two techniques suffer from practical limitations such as small field size, stitching errors, short mask lifetimes, low throughput, and high cost. Interference lithography is used in laboratory environments to produce DFB gratings with the advantages of large area, high throughput, no stitching errors and no conventional photo-masks or expensive phase masks. Additional advantages of IL technology include: a virtually unlimited field size; substrate flatness is not an issue due to the three-dimensional nature of the interference pattern; IL tools are relatively inexpensive; and the fabrication process is compatible with current semiconductor photo-processing.
Many telecommunications components manufacturers utilize benchtop IL systems configured in A division-of-wavefront arrangement for the production of the gratings within DFB lasers. FIGS. 1a and 1b depict the patterning components 10 of a division-of-wavefront IL system used to produce gratings with feature spacings suitable for DFB lasers. A single large diameter beam derived from a well polarized 32 (optimally s-polarization where the electric vector vibrates perpendicular to the plane of the drawing), temporally coherent laser source, is directed toward the intersection of two planes defined by a mirror 12 with reflective surface 14, and a workpiece 20 consisting of a substrate 18 supporting a layer of photosensitive media 16. As depicted in the figures, a portion of the beam is folded onto itself by the mirror 12. In these examples, the lower portion of the beam's wavefront 28 is incident directly upon the light sensitive material 16 at an angle θi, while the upper portion 30 is re-directed through reflection from the mirror 12 and falls upon the photosensitive medium 16 (placed in the recording plane 24) from the opposite angle −θi. An interference pattern, which has the form of a spatial modulation of the light beam in one direction, is created by the overlapping beam sections and recorded as a grating by the photosensitive medium 16 placed within the overlap volume 26. In cross section, the intensity of the interfering light varies with a sinusoidal distribution which repeats according to the classic relation:Λ=λ/n SIN(θ1+θ2)=λ/2 SIN(θi), when θ1=θ2=θi, and n=1.Here Λ is the period over which the pattern repeats, λ is the wavelength of the illumination, n is the optical index of refraction of the medium in which the beam travels, and θ1, θ2 are the angles at which the beams are incident upon the recording plane (measured with respect to a line perpendicular to the recording plane 34). FIG. 1a shows the folding mirror geometry which would lead to a pattern pitch of 270 nanometers (nm) when using laser light with a wavelength of 350.7 nanometers in the ultraviolet part of the electromagnetic spectrum. FIG. 1b shows a means for reducing this pattern pitch to 187 nm simply by changing the angle at which the light is incident upon the folding mirror 12 and workpiece 20.
To make an effective recording of the interference patterns produced by any type of IL system, the pattern must remain stationary in space during a time which is sufficient to expose the light modulation in the light sensitive material 16. This requires that all vibrations and disturbances leading to a displacement of the light sensitive material relative to the interference pattern, or causing the relative phases of the interfering optical beams to drift over the recording time, be eliminated. Due primarily to the short path lengths traveled by the split beam halves in a folding-mirror division-of-wavefront IL configuration, the passive stability of the system is far superior to the division-of-amplitude technique. However, as the size of the grating features is reduced toward the 100 nanometer sizes needed for DFB lasers operating about the 1300 nm wavelength, small mechanical and thermal disturbances in the system become significant, and consequently manufacturers find that their yields are low and that their systems require highly skilled operators. The solution to these problems is to add active stabilization to the IL system as taught in U.S. Pat. No. 5,142,385. Active stabilization requires a means for observing the relative phase drift of the interfering beam halves, and a modulator to compensate for the change. In a conventional, division-of-amplitude IL system, a transmissive electro-optic phase modulator is employed which intercepts only one of the interfering beams. This technique is not readily implemented in the folding mirror division-of-wavefront IL system due to the close proximity of the interfering beams. An alternative is to employ a moving mirror to modulate the phase of one of the interfering beams. However, traditionally a moving mirror has been considered impractical for the division of-wavefront IL system because of the large size of the mirror required. As described herein, it is an object of this invention to provide a practical method of incorporating a phase modulator in a folding mirror division-of-wavefront IL system.
The more difficult problem of obtaining the feedback signal indicating the relative phase drift in a folding mirror division-of-wavefront IL system, is not obvious and is a main object of the disclosed invention. What is needed is a means to magnify the interference pattern so that any pattern motion can be observed using an automated system such as a camera or detector pair. Prior art demonstrations have employed a grating placed in the region of space containing the interference pattern. Diffracted orders from the grating interfere at a shallow angle producing a macroscopic pattern of light and dark lines known as ‘fringes’. Motion, or a change in brightness of these macroscopic fringes relate directly to shifts in the microscopic fringe pattern. In one demonstration, this macroscopic pattern was employed strictly as a guide to align conventional photolithographic images with IL patterns, and no real time stabilization of an interference pattern was envisioned. An earlier demonstration employed the macroscopic fringes for active stabilization of a conventional division-of-amplitude IL system operating to produce a specific grating pitch. This system was limited due to the precise alignment needed to produce an observable macroscopic fringe pattern, and the need to fabricate a new grating for each pattern pitch desired. DFB laser manufacturers must vary the pitch of their gratings to change the resulting DFB laser wavelength. Thus, to satisfy DFB production goals, a practical means is needed for incorporating active stabilization into a re-configurable, production-worthy division-of-wavefront IL system.
In the fabrication of DFB lasers, the grating is produced by etching directly into the semiconductor material using an etch mask comprised of a material known as photo-resist. Photoresist is light sensitive and produces the surface relief structure needed for the etch mask. Thus photoresist is the preferred light sensitive material 16 used to record the interference pattern produced in an IL system. After exposing the photoresist in an IL system, it is processed using standard wet chemical techniques developed for the semiconductor industry. The result is an etch mask consisting of lines of photoresist material which have a cross section and spacing suitable for DFB laser fabrication. A depiction of such a grating is shown in FIG. 2. The bright lines are composed of photoresist, and these lines are supported by an indium phosphide substrate.
To produce gratings with a line spacing in the range of 130 to 190 mn, the technique known as prism coupling can be employed. Such gratings could be used to produce shorter wavelength DFB lasers for communications in local area networks and cable television, or gratings for controlling the polarization of light in display applications. The prism coupling method operates on the principle that the incident wavelength is shortened by the value of the index of refraction of the medium through which the beam passes. Referring to the equation for the grating pitch given above, note that the optical refractive index of the material in which the interfering beams travel is inversely proportional to the pattern pitch. Thus by interfering the optical beams within a higher index medium such as glass (n˜1.5), we can reduce the pattern pitch by the inverse of the glass index, 1/1.5, or 2/3. FIG. 3 depicts a folding mirror IL configuration 40 utilizing a glass prism 42 to increase the index of refraction of the medium surrounding the beam overlap volume 26. In this example, the angle of the prism face, θp is set equal to the angle of incidence of the optical beam in air θi. Once inside the prism, a reflective surface 14 applied to the back face of the prism 46, divides the illumination beam into wavefronts 28 and 30, which, as with the folding mirror IL system 10, subsequently overlap forming the interference volume 26. Optical contact or coupling, defined as the effective elimination of a material interface, is made to the workpiece 20 using a fluid 44 composed of a material with an optical refractive index which is a close match to the prism 42 material. Suitable materials are described throughout the literature, and include various forms of water, oils, solvents, and even gel materials. With this configuration and an optical illumination wavelength of 350.7 nm, the grating recorded by the light sensitive material 16, will have a line spacing of 150 nm. Again by changing the incident beam angle θi a grating pitch below 130 nm would be practical with this configuration. Note also that reflective surface 14 would become unnecessary when the incident beam angle is reduced below ˜48° in this setup (i.e. when generating gratings with line spacings between 155 and 180 nm). This is due to the total internal reflection exhibited by a properly polarized optical beam 32 traveling within a higher index material when it encounters an interface with a lower refractive index. A method of incorporating an active stabilization system which integrates a phase modulator onto the prism, would make such a prism-based division-of-wavefront system practical.
A promising technology for producing electronic paper currently under development is that of reflective LCDs. The term ‘crystal’ in liquid crystal displays refers to the structure or ordering of the LC molecules into a definable or measurable state typically found with molecules in a solid state. This artificially created ordering is accomplished by depositing thin layers of material known in the art as ‘alignment layers’, which are typically processed using a physical rubbing or buffing technique. LCD manufacturers would experience an increase in yield by incorporating a non-mechanical, non-contact alignment layer process which did not produce static charge and was compatible with existing manufacturing equipment and environments. Such a process can be realized by patterning a fine pitch grating structure into the alignment material layer using a folding mirror division-of-wavefront IL system.
Another significant use of interference lithography is in the production of flat panel displays based on distributed cathode Field Emission Display, or FED, technology. FED technology is a strong competitor for LCDs in the flat panel display market. The most critical step in the fabrication of this distributed cathode matrix of an FED is the patterning of an array of holes or wells within which each emitter cone is grown. Typically, a light sensitive material such as photoresist has been employed to record an image of a hole array formed by lithographic techniques such as shadow masking (contact printing), optical projection, or electron beam writing. The hole array in photoresist then acts as an etch mask in the process of forming the holes. These patterns are limited by the resolution and field size of the imaging or writing systems, and complex, often expensive, workaround solutions are required to achieve modest field sizes of 50×50 mm with hole diameters in the 1 to 2 micron range. Recent work by many researchers has demonstrated that a reduction in the hole size (and consequently the emitter size), below the one micron range provides numerous benefits such as a reduced gate voltage, lower power consumption, greater current densities per pixel, and a built-in redundancy. Thus, to fully realize the potential of FED technology, an inexpensive, high speed, production environment lithographic system is needed which can produce sub-micron diameter hole arrays in large areas with few defects and low cost.
Other notable surface relief structures which can be patterned using IL, are known as ‘motheye’ or sub-wavelength-structure(SWS) surfaces. These surfaces have been shown to be effective at nearly eliminating the reflectance of light from an optical interface such as windows and refractive optical elements. To avoid diffraction effects, motheye surfaces must be generated with feature sizes and spacings smaller than the wavelength of light which will be employed, and the surface textures must vary in two dimensions to avoid polarization affects. For most infrared or visible wavelength applications, this necessitates structure sizes in the sub-micron to sub-half-micron range patterned over the entire window or optic area. A means for manufacturing motheye structures in high volumes and over large-areas is not currently available. An actively stabilized two-mirror division-of-wavefront IL system as disclosed herein, would make the production of motheye structures practical.
Interference lithography demonstrations in the laboratory have typically employed UV wavelength light derived from an argon ion gas laser. These gas lasers are highly automated and reliable making them a good choice for a manufacturing system. A wavelength in the deep blue is also available with these lasers, and the wavelength choice becomes a tradeoff between photoresist sensitivity and laser power. A large variety of photoresists possess high sensitivity to near UV light, known in the semiconductor industry as the ‘i-line’, whereas the number of blue wavelength (‘g-line’) sensitive photoresists is more limited, and the blue sensitivity is typically much lower. Many of the driving applications for the division-of-wavefront IL technology described above require nanometer scale feature sizes. Because the feature size produced in an IL system is directly related to the laser wavelength, using a near UV wavelength of 350.7 or 364.8 mn, results in 25% smaller feature sizes than an IL system operating in the blue at 442 or 458 nm. Thus an IL system operating in the UV can achieve the 200 nm grating pitch needed for DFB laser production, without resorting to exotic techniques such as prism coupling. However, there are several applications where gratings with feature sizes in the 60-80 nm range are required. For example, wire-grid polarizers designed to operate over the visible spectrum (400-700 nm) are constructed from gratings with pitches around 150 nm. As described above, the folding mirror division-of-wavefront IL system 40 shown in FIG. 3, can be adapted to pattern 150 mn pitch gratings through use of prism coupling techniques, or by utilizing a shorter wavelength UV source. The latter approach has become more feasible with the introduction of compact solid state sources such as the quadrupled Nd:YAG system (266 nm wavelength) manufactured by laser producers such as Coherent and Spectra-Physics.
Considering all of the potential applications of interference lithographic structures mentioned above, there is a need for a production device capable of making these structures in a fast and economical fashion.